This invention relates to the field of diagnostic ultrasound, and, more particularly, to a method and system that can accurately estimate the flow volume of a vessel without requiring knowledge of the vessel shape or angular orientation.
Ultrasound can be used to image tissues and vessels using a variety of imaging modalities. For example, B-mode scanning can be used to image tissues by portraying the tissues in a gray scale in which the brightness of each region in the image is a function of the intensity of ultrasound returns from corresponding regions of the tissues. B-mode scanning can be used to visualize the shapes of organs and vessels, and to detect the presence of masses, such as tumors, in tissues.
Doppler scanning can be used to provide images showing the velocity of moving sound scatterers, such as blood flowing through an artery or vein. Using Doppler scanning to image the flow pattern of blood through a vessel allows the blood flow velocities and the internal shape of the vessel to be imaged and quantified. As a result, stenoses due to partial obstructions in blood vessels can be detected.
Attempts have also been made to measure volume flow rate, i.e., the volume of fluid flow, using Doppler scanning. Volume flow measurements can be important to determine, for example, cardiac output. In one prior art approach, a phased array scanhead is used to obtain a plurality of spaced-apart Doppler scan lines in a cross-section of the vessel. The scanlines are then combined to create a two-dimensional image depicting the velocity of blood flow through the vessel. The velocity of blood flowing through the vessel can then be integrated across the area of the vessel to determine the volume flow rate. To operate from multiple positions in an acceptable timeframe, the number of scan planes must be limited to a value that results in the scan planes being significantly spaced apart from each other. As a result, much of the flow information depicted in the image area is obtained by interpolating between adjacent planes. Since blood flow through a vessel can be highly irregular, there can be no assurance that the mean velocity values accurately depict the actual blood flow in the regions between the scan planes. Furthermore, measured flow velocity differs from the actual flow velocity by the cosine of the angle between the ultrasound beam and the flow direction. Yet is often difficult to accurately determine the direction of blood flow. Thus, conventional Doppler imaging using a two dimensional approach often does not provide an accurate depiction of blood flow.
As previously mentioned, volume flow rate of blood in a vessel is the blood flow velocity integrated across the area of the blood vessel. Thus, inaccuracies in measuring blood velocity using conventional, two dimensional Doppler imaging correspondingly affect the accuracy of blood volume flow rate determinations.
The above-described limitations in measuring blood volume flow using conventional, multiple scanline Doppler imaging techniques has resulted in the development of improved techniques for measuring and depicting blood flow rate. According to one technique described in U.S. Pat. No. 5,623,930 to Wright et al., blood flow velocity through a vessel is measured in several planes intersecting the vessel at different angles. By processing this blood flow velocity data, blood volume flow can be determined without knowing the angle between the ultrasound beam and the direction of blood flow. However, the needed geometric assumption and the need to obtain blood velocity measurements in several planes would appear to limit the usefulness of this technique.
Another approach described in publications by Kim et al., entitled xe2x80x9cA New Doppler Method for Quantification of Volumetric Flow: In Vivo Validation Using Color Doppler,xe2x80x9d Journal of the American College of Cardiology, 1996, and by Brandberg et al., entitled xe2x80x9cIncreased Accuracy of Echocardiographic Measurement of Flow Using Automated Spherical Integration of Multiple Plane Velocity Vectors,xe2x80x9d Ultrasound In Med. and Biol., 1999, is able to directly measure and image blood volume flow rate without the need to know the angle between the ultrasound beam and the blood flow direction. With reference to FIG. 1, the volume flow rate of blood through a blood vessel 10 can be measured by measuring the volume flow rate through any arbitrary sample surface 14 passing through the vessel. The volume flow rate through the sample surface 14 can be measured by first determining the velocity of blood flowing through the sample surface 14 by performing a three-dimensional Doppler scan. The velocity is then integrated throughout the area of the sample surface 14.
The reason the surface 14 need not be particularly oriented is that whatever volume of blood flows through any surface or plane extending through the vessel 10 also flows through the sample surface 14. Thus, the sample surface 14 can be any arbitrary shape having any arbitrary orientation to the flow of blood through the vessel 10. In practice, as shown in FIG. 2, a spherical sample surface 20 is obtained by obtaining a three-dimensional Doppler image in a narrow sample volume 22 equidistant from a two-dimensional array scanhead 24. A Doppler scan of this type is in this context referred to as Flow-mode, or F-mode, scanning. A 3-D flow image 26 obtained by an F-mode scan is shown in FIG. 3. As shown therein, the 3-D flow image 26 is rendered as a spherical cross section 28 through the blood vessel 10 corresponding to the spherical sample surface 20. Although the flow rate of blood can be determined using F-mode scanning, it is difficult to portray the flow velocity in the spherical cross-section 28. Among other factors, it is difficult to render a three-dimensional surface in the two-dimensions available in a conventional display. The need to render an image in three dimensions also requires a relatively large amount of processing time, thus making it difficult to provide real time volume flow images. Furthermore, it is more complicated to compute the data on the spherical surface along with the desired user interactions indicating the flow region of interest.
Another difficulty with the flow imaging technique exemplified by FIG. 2 is delineating the boundary between the blood flow and the wall of the vessel 10. This process, known as segmentation, is important for defining the area over which the flow velocity will be integrated. If segmentation does not include all of the blood flow, the volume flow measurements will be inaccurate. If the segmentation includes surrounding flow events, integration will occur over areas that need not be integrated, thereby increasing the acquisition and processing time needed to determine blood volume flow rate. Also, if the segmentation encompasses an area occupied by other blood vessels, volume flow calculations may be inaccurate. Segmentation is currently accomplished in most cases by identifying control points on the boundary between the blood flow and the vessel 10. The ultrasound monitor then connects the control points to complete the segmentation process. Accurate segmentation requires a large number of control points, thus making this segmentation technique very time consuming. Also, it can be difficult to accurately identify the boundary between the blood flow and the vessel wall.
There is therefore a need for a method and apparatus that can accurately portray blood flow in two dimensions so that volume flow rate can be easily and quickly calculated, and which can quickly and easily segment the image to delineate the boundary between blood flow and vessel walls.
A method and system for generating an ultrasound image shows the velocity of blood flowing through a blood vessel and calculates the volume flow of the blood. A two-dimensional ultrasound transducer array or a moving one-dimensional array is used to obtain a three-dimensional Doppler image of blood flow velocity in a relatively narrow measurement volume. The measurement volume intersects the blood vessel a fixed distance from the transducer. The Doppler data of the measurement volume is then electronically projected onto a planar surface to create a two-dimensional blood flow velocity image that can be viewed on a display. The volume flow rate of the blood flowing through the vessel can be determined by integrating the blood flow velocity within the two-dimensional blood flow velocity image. A cross section of the blood vessel obtained by B-mode scanning can be superimposed on the two-dimensional blood flow velocity image.